ADS1610
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SBAS344C–AUGUST 2005–REVISED OCTOBER 2006
FREQUENCY RESPONSE
0
1
2
3
4
5
6
7
The linear phase FIR digital filter sets the overall
frequency response. The decimation rate is set to 6
(MODE = 00) for all the figures shown in this section.
Figure 34 shows the frequency response from DC to
30 MHz for fCLK = 60 MHz. The frequency response
of the ADS1610 filter scales directly with CLK
frequency. For example, if the CLK frequency is
decreased by half (to 30 MHz), the values on the
X-axis in Figure 34 would need to be scaled by half,
with the span becoming DC to 15MHz.
−
−
−
−
−
−
−
4.0 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 4.9 5.0
Frequency (MHz)
0
−
−
−
−
20
40
60
80
Figure 36. Passband Transition
The overall frequency response repeats at multiples
of the CLK frequency. To help illustrate this,
Figure 37 shows the response out to 180 MHz (fCLK
=
60MHz). Notice how the passband response repeats
at 60MHz, 120MHz, and 180MHz; it is important to
consider this sequence when there is high-frequency
noise present with the signal. The modulator
bandwidth extends to 100MHz. High-frequency noise
around 60MHz and 120MHz will not be attenuated
by either the modulator or the digital filter. This noise
will alias back inband and reduce the overall SNR
performance unless it is filtered out prior to the
ADS1610. To prevent this, place an anti-alias filter in
front of the ADS1610 that rolls off before 55MHz.
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−
100
120
0
5
10
15
20
25
30
Frequency (MHz)
Figure 34. Frequency Response
Figure 35 shows the passband ripple from DC to
4.4MHz (fCLK = 60MHz). Figure 36 shows a closer
view of the passband transition by plotting the
response from 4.0MHz to 5.0MHz (fCLK = 60MHz).
0
−
−
−
−
20
40
60
80
0.00020
0.00015
0.00010
0.00005
0
−
100
120
−
−0.00005
−0.00010
−0.00015
−0.00020
0
20
40
60
80
100 120 140 160 180
Frequency (MHz)
Figure 37. Frequency Response Out to 120MHz
0
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
Frequency (MHz)
Figure 35. Passband Ripple
17
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